From the article “Ultraschallsensoren für die Prozesstechnik” by Alf Püttmer, Edmund Linzenkirchner and Peter Hauptmann, published in atp (Automatisierungstechnische Praxis), 46 (2004), issue 1, pages 51 to 59, various application principles are known for ultrasound sensors when measuring physical or chemical values, such as temperature, pressure, throughflow, concentration and density. The use of ultrasonic transducers has the advantage that the ultrasonic waves penetrate through a container or tube wall and into the medium to be examined and therefore allow non-invasive measurement of the various values. The ultrasonic waves are emitted and received after interacting with the medium to be examined. They then carry information about the parameters to be measured. Sound generation and sound reception in process applications are usually carried out using piezoelectric sound transducers. In addition to the sound transducers an activating and analytical device is also required with which signal generation in the transmitting transducer is initiated, the electrical output signal of the receiving transducer detected and a signal analysis carried out. In an ultrasonic flowmeter two sound transducers, which are alternately called a transmitting transducer and a receiving transducer, are arranged on a measuring tube so as to be axially spaced apart. The propagation time difference method utilizes the carrying effect of a sound pulse with the flow inside the tube. This results in different propagation times, both in and counter to the direction of flow, from which the flow speed is calculated. The method is used in liquids and gases. With knowledge of the density of the medium and the geometry of the measuring tube the mass flow can be determined using the measured flow speed. In the case of media with changeable densities, the current density of the medium also has to be determined therefore to calculate the mass flow. This can take place using a separate ultrasonic density sensor. In principle the density of the medium could be inferred using an amplitude measurement of the received signal. The sound absorption depends on various properties of the media however, such as density, viscosity, thermal conductivity, thermal capacity or thermal coefficient of expansion. Determining the density using an amplitude measurement is therefore subject to a high level of inaccuracy owing to the strong cross-sensitivities.
An ultrasonic flowmeter is known for example from DE 196 11 233 A1. In this case a transmitting transducer emits pulsed signals which reach the receiving transducer as a wave group that increases and decreases again over a plurality of oscillations.